Separate Confinement Heterostructure Quantum Research Articles

A Compact High-Efficient Equivalent Circuit Model of Multi-Quantum-Well Vertical-Cavity Surface-Emitting Lasers for High-Speed Interconnects

Due to their low power consumption, high modulation speed, and low cost, vertical-cavity surface-emitting lasers (VCSEL) dominate short-reach data communications as the light source. In this paper, we propose a compact equivalent circuit model with noise effects for high-speed multi-quantum-well (MQW) VCSELs. The model comprehensively accounts for the carrier and photons dynamisms of a MQW structure, which includes separate confinement heterostructure (SCH) layers, barrier (B) layers, and quantum well (QW) layers. The proposed model is generalized to various VCSEL designs and accommodates a flexible number of quantum wells. Experimental validation of the model is performed at 25 Gb/s with a self-wire-bonded 850 nm VCSEL.

Nanoscale effects on the thermal and mechanical properties of AlGaAs/GaAs quantum well laser diodes: influence on the catastrophic optical damage

In this work we study the catastrophic optical damage (COD) of graded-index separate confinement heterostructure quantum well (QW) laser diodes based on AlGaAs/GaAs. The emphasis is placed on the impact that the nanoscale physical properties have on the operation and degradation of the active layers of these devices. When these laser diodes run in continuous-wave mode with high internal optical power densities, the QW and guide layers can experiment very intense local heating phenomena that lead to device failure. A thermo-mechanical model has been set up to study the mechanism of degradation. This model has been solved by applying finite element methods. A variety of physical factors related to the materials properties, which play a paramount role in the laser degradation process, have been considered. Among these, the reduced thicknesses of the QW and the guides lead to thermal conductivities smaller than the bulk figures, which are further reduced as extended defects develop in these layers. This results in a progressively deteriorating thermal management in the device. To the best of our knowledge, this model for laser diodes is the first one to have taken into account low scale mechanical effects that result in enhanced strengths in the structural layers. Moreover, the consequences of these conflicting size-dependent properties on the thermo-mechanical behaviour on the route to COD are examined. Subsequently, this approach opens the possibility of taking advantage of these properties in order to design robust diode lasers (or other types of power devices) in a controlled manner.

The optical gain and radiative current density of GaInNAs/GaAs/AlGaAs separate confinement heterostructure quantum well lasers

The optical gain and radiative current density of GaInNAs/GaAs/AlGaAs separate confinement heterostructure quantum well (QW) lasers with an emission wavelength of 1.3 μm have been theoretically investigated. The effect of carrier leakage from the GaInNAs QW to the GaAs waveguide layer is studied, and its influence on the optical gain and radiative current density is identified. The hole filling caused by an injected carrier has a strong impact on the optical gain and radiative current density, while the effect of electron filling is negligible, reflecting the smaller band-gap discontinuity in the valence band than in the conduction band. Hole occupation in the waveguide layer decreases the optical gain, and increases the radiative and threshold current densities of the laser. Our calculated threshold current density (659.6 A/cm2) at T=300 K is in good agreement with the experimental value (650.9 A/cm2) reported in literature [R. Fehse et al., IEEE J. Sel. Top. Quantum Electron. 8, 801 (2002)].

An Equivalent Circuit Model for Analyzing Separate Confinement Heterostructure Quantum Well Laser Diodes Including Chirp and Carrier Transport Effects

In this article, we derive a new SPICE equivalent-circuit model, using the rate equations for a separate confinement heterostructure quantum well laser. In this model, based on physical principles, the frequency chirping phenomenon has been incorporated by the introduction of an exact carrier-dependent linewidth enhancement factor for quantum well lasers. The effects of different separate confinement heterostructure layer lengths and carrier transport effects on the transient and modulation responses are also analyzed through this new model.

MOCVD growth of 980 nm InGaAs/GaAs/AlGaAs graded index separate confinement heterostructure quantum well lasers with tertiarybutylarsine

InGaAs/GaAs/AlGaAs quantum well (QW) structures have been grown at different temperatures by metalorganic chemical vapor deposition (MOCVD) with tertiarybutylarsine (TBAs) as the group V source, and it is found that the QW structures grown at 640 °C shows the strongest photoluminescence (PL) emission. Based on the optimized growth conditions, the InGaAs/GaAs/AlGaAs graded index (GRIN) separate confinement heterostructure (SCH) double quantum well lasers have been grown with TBAs. The 4-μm-wide ridge waveguide lasers show room temperature continuous-wave lasing around 980 nm, with a typical threshold current of 14 mA, indicating that TBAs could be a promising alternative to the highly hazardous gas source AsH 3 in growing InGaAs/GaAs/AlGaAs 980 nm QW lasers.

Growth and optimization of extremely high-pulse-power graded-index separate confinement heterostructure quantum well AlGaAs/InGaAs diode lasers with broadened waveguides

Material quality is an essential prerequisite and a major challenge for the fabrication of high-power, 980-nm, strained-quantum-well (SQW) InGaAs lasers. We report our work aimed at metal-organic chemical vapor deposition (MOCVD) growth optimization and epitaxial quality analysis of various graded-index separate confinement heterostructure (GRINSCH) QW AlGaAs/InGaAs laser structures. Systematic investigation of doping level control and minimization of oxygen incorporation in AlGaAs were performed. Background oxygen levels of 1015 cm−3 were obtained with n-(Si) and p-(C) doping concentrations as high as 1 × 1018 cm−3 and 3 × 1018 cm−3, respectively, for Al0.4Ga0.6As layers. Double-crystal x-ray (DCXR), room-temperature photoluminescence (PL) mapping, Hall effect measurements, and secondary ion-mass spectroscopy (SIMS) techniques were used to evaluate material quality. A record, multimode, pulsed output power of 52.1 W has been obtained from 100-µm × 2-mm broad-stripe lasers made from these materials. The devices demonstrate low threshold current, low cavity losses, and kink-free light-current characteristics.

A novel waveguide structure to reduce beam divergence and threshold current in GaInP/AlGaInP visible quantum-well lasers

A novel waveguide structure for AlGaInP visible separate confinement heterostructure quantum well laser with inserting low-refractive-index AlAs layers is theoretically investigated using the transfer matrix method. By using this structure, we can significantly improve the transverse beam divergence and reduce the threshold current. Otherwise, the inserting AlAs can also be used as a wet etching automatic stopped layer. Because the thermal resistance of AlAs is smaller than that of AlGaInP, the thermal characteristics of this novel structure are also better than the conventional AlGaInP visible lasers. With the inserting AlAs layers, the transverse beam divergence and the threshold current density can be reduced from 38° to 8.84° and 787 to 666.8 A/cm2, respectively.

Simple analysis of carrier transport and buildup in separate confinement heterostructure quantum well lasers

A simple, analytical model for carrier transport and buildup in the separate confinement layer of a quantum well laser is developed. The time constant characterizing the low-frequency roll-off is separated into components describing electron and hole transport and components related to the charge storage effects arising from electron and hole capture. In contrast to previous work, the analysis shows that both the transport and capture related delays are sensitive to the placement of the well within the separate confinement heterostructure and that the carrier concentrations change abruptly across the quantum well. >

Monte Carlo analysis of the carrier relaxation processes in linear- and parabolic-GRINSCH quantum well laser structures

The carrier relaxation process is widely acknowledged to have a strong bearing on the modulation limit of quantum well lasers. As a first and crucial step toward achieving a better understanding of this phenomenon, we have developed a numerical technique to study such processes in graded-index separate confinement heterostructure quantum well laser structures having an arbitrary grading profile. We base our approach on ensemble Monte Carlo simulation of the carrier transport in the 3-D graded-index region and in the 2-D quantum well. We also introduce a technique to handle the carrier capture and re-emission processes within the Monte Carlo method. The results obtained from our calculations for a number of structures with quantum well sizes 50-100 /spl Aring/ indicate that the overall carrier capture time is about 5-7.5 ps under low injection condition for the linearly graded structures, and significantly longer for the parabolically graded structures. On the other hand, the carrier capture efficiency is found to be higher for the parabolic graded-index structures. We also compare our calculations to published experiments and find good agreement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Growth of InAIGaAs strained quantum well structures for reliable 0.8 μm lasers

Incorporation of indium into the quantum well materials of graded-index separate confinement heterostructure quantum well lasers has proven to be a key to imparting a much needed robustness to such lasers. By growing wells which contain both indium and aluminum along with gallium, operating wavelengths can be engineered to fall in the technologically important range of 0.8 microns, appropriate for pumping Nd:YAG. The organometallic vapor phase epitaxial growth of these strained-layer structures faces extra challenges rooted in the competing influences on the energies of the quantized states. At a minimum, meeting wavelength targets requires achieving control of the quaternary composition and of the quantum well thickness. Because laser elements are relatively large, lateral uniformity of wavelength is a critical issue. Device performance is influenced by basic material quality, which is a function of such fundamental growth parameters as temperature, V/III ratio, and growth rate. We have grown InAIGaAs structures using various combinations of growth conditions and well composition and thickness combinations, and evaluated and life-tested lasers in CW mode. The reactor’s performance in achieving composition and thickness uniformity is reported, as are data on the influence of the effects of growth conditions on device performance.

Monte Carlo simulation of gain compression effects in GRINSCH quantum well laser structures

Gain compression is widely acknowledged to be a serious limitation to the ultimate modulation bandwidth of a semiconductor laser. We have developed a numerical technique to study the gain compression effects in graded-index separate confinement heterostructure (GRINSCH) quantum well laser structures, This technique is based on the combination of the Monte Carlo simulation of the carrier dynamics in the device while under intense stimulated photon emission, and the calculation of the optical gain using a 4/spl times/4 k/spl middot/p Hamiltonian. From the simulated results, we calculated a gain compression coefficient /spl epsiv/=1.1/spl times/10/sup -17/ cm/sup 3/ for a linearly graded quantum well laser structure having a 50 /spl Aring/ In/sub 0.2/Ga/sub 0.8/As well. We find good agreement between our results and published experiments. We have also demonstrated that our calculation method is capable of simulating the gain dynamics in the laser structure, such as those studied with femtosecond pump-probe experimental techniques. >

Improvement of GaAs/AlGaAs quantum well laser diodes by rapid thermal annealing

Post-growth annealing is shown to improve the laser diode quality of GaAs/AlGaAs graded-index separate confinement heterostructure quantum well laser diode structures grown at a nonoptimal substrate temperature lower than 680°C by molecular beam epitaxy. Reduction by a factor of up to three in the threshold current was accompanied by a reduction in the interface trap density. The reduced threshold current is still higher than that of laser diodes grown at the optimal temperatures which are between 680 and 695°C. The improvement in laser diode performance is ascribed to the reduction of interface nonradiative recombination centers.

Monte Carlo studies on the well-width dependence of carrier capture time in graded-index separate confinement heterostructure quantum well laser structures

The total carrier capture time and the quantum well width are both important parameters affecting the graded-index separate confinement heterostructure (GRINSCH) quantum well laser modulation speed limit. However, discrepancies exist in the literature on the well-width dependence of the carrier capture times. To study this phenomenon, we have developed a Monte Carlo technique to simulate carrier relaxation in GRINSCH quantum well structures. Our results show that the carrier capture time increases with the density of carrier injection. Furthermore, depending on the concentration of injected carriers, the capture time will either decrease, remain the same, or increase with increases in the well width. At lasing conditions, the times are more or less independent of the well width up to 100 Å. We compare our calculations to published experiments and find good agreements.

On the effects of carrier diffusion and quantum capture in high speed modulation of quantum well lasers

It is shown that, the combined effects of carrier diffusion in the separate confinement heterostructure region and quantum capture into the quantum well must be considered together when evaluating the limits on the modulation bandwidth of quantum well lasers, despite the fact that quantum capture is considerably faster than carrier diffusion. The importance of quantum capture may be observed by noting that the modulation bandwidth is adversely affected even for quantum capture times as short as 0.2 ps, or if the ratio of the quantum capture time to quantum escape time is large (≳0.01). Therefore, the bandwidth limitation may be caused by electron transport rather than hole transport since quantum capture of holes is faster than that of electrons.

Secondary ion mass spectrometry analysis of Be doped GaAs/AlGaAs heterostructures

Secondary ion mass spectrometry (SIMS) has been used to study the depth profiles of beryllium (Be) incorporation and diffusion in GaAs/AlGaAs heterostructures of graded index separate confinement heterostructure quantum well lasers. The epilayers were grown by molecular‐beam epitaxy with a thermally cracked As2 source. The GaAs quantum well was Be‐doped with Be at 2×1019 cm−3 using a substrate growth temperature of 580 °C and the Al0.4Ga0.6As confinement layers were grown at 630 °C. SIMS measurements show negligible diffusion of Be under these growth conditions. SIMS depth profiling was also used to resolve thin (2 nm) GaAs smoothing layers incorporated in Al0.4Ga0.6As confinement layers.

Effect of design variations on the threshold current density of AlxGa1−xAs separate confinement heterostructure single quantum well lasers

Data from a series of separate confinement heterostructure quantum well AlxGa1−xAs lasers of various structural design are analyzed so as to define material and device parameters suitable for subsequent modeling and threshold current optimization of arbitrary separate confinement laser structures. By modeling variations in inner barrier and cladding layer compositions, inner barrier width, and quantum well size, we show that, through proper design, low threshold current densities can be realized for structures having both indirect and direct barriers, further demonstrating that electron confinement due to a separate confinement heterostructure has no effect on carrier collection in the quantum well active layer. We also demonstrate the insensitivity of threshold current density to deviations in barrier width from an optimum value due to simultaneously induced variations in the optical confinement factor and overall optical loss coefficient.

Interface traps and interface recombination in AlGaAs/GaAs quantum well laser diodes

The current-voltage characteristics and deep traps of various GaAs/AlGaAs graded-index separate confinement heterostructure quantum well laser diode structures are studied as a function of the growth temperature and threshold current. It is shown that the interface nonradiative recombination processes cause a high threshold current. An impurity-related deep level with activation energy Ea=0.48 eV at the interface region and an interface state with wide energy distribution were found in the high threshold current diodes. The current-voltage characteristics show that interface recombination, rather than bulk recombination, is the dominant carrier transport process in the diodes and is responsible for the high threshold current.

Analysis of current injection efficiency of separate-confinement-heterostructure quantum-film lasers

Current injection efficiency, i.e. the proportion of current into the active region to total current, is analyzed for separate confinement heterostructure (SCH) quantum film lasers. It is shown that the current injection efficiency changes stepwise with the active layer thickness. and is larger for lower injected carrier density and deeper quantum well. Comparison is made between step-, parabolic-GRIN-, and linear-GRIN-SCH structures. The efficiency of linear-GRIN-SCH is the highest for the same well depth. Threshold current density is discussed for these SCH structures, taking into account the current injection efficiency and the optical loss due to the carrier leakage to the optical confinement layers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Interface Recombination and Threshold Current in Grinsch-QW ALGaAs/GaAs Laser Diodes

ABSTRACTA series of Graded-Index Waveguide Separate-Confinement Heterostructure Quantum Well (GRINSCH-QW) laser diodes were grown by MBE at the systematically varied substrate temperatures. The threshold current of laser diodes were found to depend strongly on the growth temperature. The structure and electrical characteristics of the laser diodes were studied by double-crystal x-ray diffraction, I-V-T, C-V and deep level transient spectroscopy (DLTS). The interface recombination is found to be the dominant carrier transport process in the high threshold current laser diodes and is closely related to the presence of the high concentration of deep traps and interface states. In the low threshold current laser diodes, diffusion process is found to be the dominant carrier transport process.

Measurement of the ambipolar carrier capture time in a GaAs/Al xGa 1−xAs separate confinement heterostructure quantum well

The carrier capture in a separate confinement heterostructure quantum well has been studied both experimentally and theoretically. Our calculations show that the electron and hole capture time vary strongly as a function of the excess energy. At an excess energy of 40 meV, both capture times are equal resulting in an ambipolar capture process which allows a direct comparison between theory and experiment. We carried out subpicosecond luminescence spectroscopy experiments and deduce an ambipolar overall capture time of 20 ps, a number which for the first time is in agreement with theoretical predictions. The quantum mechanical overall capture time of 20 ps gives rise to a classical local capture time of 3 ps which is determined from a diffusion model.